The present invention relates to a novel thermosetting polyimide resin composition. More concretely, the present invention relates to the thermosetting unsaturated bismaleimide resin composition possessing excellent processability and heat resistance, lower mold shrinkage, and good external appearance of its molded product.
The thermosetting resin has been widely used in particularly the heat resistance-required fields such as the electronics industry, the auto industry, the machine-parts industry, and the like. And, there have been constant efforts for giving good mold ability to such thermosetting resin and reducing the mold shrinkage thereof.
In the case that the thermosetting resin is used as sealing materials for semiconductor devices, there are particularly required better heat resistance, improved impact strength, good damp resistance and low mold shrinkage. Recently, the demand for the unsaturated bismaleimide resins which have especially good heat resistance has rapidly increased.
The unsaturated resins produced by polymerization of only bismaleimides themselves, have generally distinguished heat resistance to the extent enough to show 430.degree. C. or more of the thermal dissociation temperature because of their high crosslinking densities. On the other hand, they have some disadvantages in that they show inferior toughness because of their poor plasticities and they can be dissolved in only polar solvents having high boiling points such as N-methyl-2-pyrrolidone (NMP, b.p. 202.degree. C.), N,N'-dimethyl formamide (DMF, b.p. 153.degree. C.), and the like. Moreover, such resins require a long time for molding-processing because of their high curing temperatures, therefore, they have also an additional disadvantage in practical use.
U.S. Pat. No. 3,562,223, discloses a method that a modified bismaleimide resin is prepared by the Michael addition reaction of unsaturated bismaleimides and primary diamines. In the patent, the crosslinking density of the resin when cured, is reduced, and consequently the heat resistance of the resultant modified denatured bismaleimide resin deteriorates a little, but the resin has a toughness to some extent. And, there is used an organic peroxide as the curing accelerator in order to shorten the curing time.
Typical modified bismaleimide resin's compositions as plastic materials which are on the market, are the Kinel series manufactured by Rhone Poulenc Co. in France. They are thermosetting resin compositions composed of polyamine bismaleimide (PABM) prepolymer having a molecular weight of about 1,000, inorganic fillers and the other additives, wherein the prepolymer may be prepared by the Michael addition of N,N'-4,4'-diphenylmethane bismaleimide as a bismaleimide and 4,4'-diamino diphenyl methane as a primary diamine.
However, when said polyamine bismaleimide (PABM) is prepared in accordance with the method as disclosed above, there are some disadvantages, that is, its viscosity increases due to the progress of partial crosslinking, and thus the prepared polyamine bismaleimide becomes soluble only in a polar solvent having a high boiling point, such as N-methyl-2-pyrrolidone, etc. Also, because there is used an organic peroxide as the curing accelerator, gas is generated from thermal dissociation of the organic peroxide during molding process, and then the external appearance of the molded product cannot be preserved from damages.